A new artificial muscle made of polymer can contract forcefully when it reaches approximately 42 °C, a temperature close to that of the human body.
This material was designed by researchers from Donghua University in Shanghai, together with a scientist from Forschungszentrum Jülich in Germany. It belongs to the family of shape-memory polymers. These materials can change shape in response to a stimulus and then return to a specific configuration. Here, the researchers use both heat and water to control the movement.
Pixabay illustration
To understand how it works, we need to examine the material's structure at a very small scale. The researchers combined long chains of polyacrylic acid with a compound called a surfactant. Some areas attract water, while others respond mainly to temperature.
Heat acts like a switch. The surfactant changes its organization around 42 °C, triggering the polymer's movement. This temperature remains moderate for a thermally activated material. It therefore makes it possible to obtain contraction without having to heat the device to much higher temperatures.
The measurements provide an idea of the force produced. When the material is prevented from moving, it can exert a stress of up to 6.4 megapascals. This value measures the force relative to the material's surface area. Without a load, its length can vary by up to 79%. These performances were measured while heating the material to around 42 °C.
In other words, the same material can produce significant force or substantial displacement, depending on how it is used. The researchers also report reversible cyclic movements without applying an external force to reset the system. An artificial muscle must be able to repeat its contractions to become useful in a machine.
This type of material is of particular interest to soft robotics. Conventional robots often use motors and rigid mechanisms to create movement. A polymer capable of contracting directly could enable more flexible devices, particularly for grippers, wearable systems, or certain machines inspired by the human body. However, the study focuses on an experimental material, not on a device ready for commercialization.
The researchers now have a method for separately adjusting the force and range of movement through the polymer's internal organization. This approach could help design other soft actuators suited to specific applications. It remains necessary to determine, in particular, how these materials behave over long periods and in complete devices.